Some years ago, on a different board, I recall a long and heated debate about this very subject in which someone very stridently insisted that the dream HAD to come from Ulmo, because only Ulmo still cared enough about ME to remain involved in its affairs. This argument was supported by the person's contention that all prophetic dreams during LotR supposedly took place near water, so naturally, Ulmo would be the person who sent them. Some people accepted this as proof, others didn't. I didn't. Not that I don't believe that Ulmo COULD have sent it, but because I don't believe the other Valar were as totally uninvolved with ME as it would appear on a casual glance. In his battle with Shelob, Sam calls the name of Elbereth, is suddenly moved to call out to her for help in a language he doesn't even know, and in response, the light of the Phial blazes "like a star that leaping from the firmament sears the dark air with intolerable light," blinding Shelob so that recoils and finally retreats, giving Sam a last shot at her legs. And then there are the Eagles, which conveniently appear to show up when Gandalf, a servant of Manwe, needs them most. I suspect the Valar haven't totally withdrawn their interest, or their help; they only do so in moments of utmost need, and then as subtly as possible.
Personally, I have long thought that it's possible the dream came from Gandalf -- who was the Counsellor of Irmo, the Master of Dreams. He knew about the Sword That Was Broken, and where it dwelt, the fact that Elrond was a master of Lore, in whose house crucial matters were often taken in council (as with the White Council); he already knew about Isildur's Bane, as well as the importance of the Halfling; moreover, he had already performed the test on the Ring and told Frodo to head for Rivendell before Faramir and Boromir had the dream, which first came on the night before Sauron attacked Osgiliath on June 20th (according to Boromir's report to the Council). If Gandalf -- who, as the Maia Olorin, was responsible for giving helpful, if anonymous, visions to the Eldar of earlier ages -- was not the one who sent the dream, it might well have been Irmo, from whom he probably learned much about such matters. I certainly don't believe either of them were responsible for all such things in LotR (Frodo's dream of Gandalf imprisoned in Orthanc certainly didn't come from him, nor, I think, from Irmo), but the dream sent to Faramir and Boromir was so specific in what it revealed and so direct in to whom it went, I believe it was intended to help prompt the Gondorians to stop looking at the welfare and defense of Gondor alone, and start realizing that there was still a larger world outside her borders, in which important things were happening and of which they should be a part. No man is an island, as the saying goes, and Gondor under Denethor's rule was coming perilously close to becoming one, consoled by the thought (which Denethor promulgated) that if Gondor fell, so would the rest of ME. The dream, if one wants to think of it in political terms, was rather anti-isolationist, and attempting to unify the free peoples in the fight against Sauron was larger mission of the Istari.
Oh, my, that was quite a ramble. Blame it on new meds that make me so sleepy....
